Rear projection screen displays based on glass microspheres embedded in an opaque matrix as described in U.S. Pat. No. 2,378,252 (Staehle) have been growing in popularity for various uses, such as in large format televisions. A rear projection screen is a sheet-like optical device having a relatively thin viewing layer that is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by a projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by a projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
A well-known type of rear projection screen is a thin, light diffusing layer such as a frosted or translucent glass surface, which may be produced by etching, sandblasting, or otherwise roughening a smooth glass surface. The translucent surface limits the visibility of objects behind the screen. The screen must, however, be sufficiently light transmissive to allow the projected image, which is focused precisely on the translucent surface, to be viewed from the front side of the screen. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent suffer, however, from a tendency to strongly reflect ambient light incident on the front side, thereby causing fading, or washout, of the projected image. This problem is particularly severe if the background or ambient light is bright.
An approach to reducing the effects of ambient light while still maintaining an acceptable level of projected image light is to attach an array of closely packed glass microspheres (i.e., beads) to a substrate by an opaque polymeric binder. The glass microspheres and substrate are both light transmissible (i.e., transparent). The glass microspheres act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the microspheres. The foci are approximately in the areas where the microspheres contact the front support layer.
Because the transparent microspheres contact the front of the substrate, they exclude most of the opaque binder material from the space between the microspheres and their contact areas on the substrate. This forms an optical aperture between each microsphere and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the microsphere interstices. As a result, ambient light incident in these areas is absorbed. Thus the front side of the screen appears black, except for the light transmitted through the microspheres.
The appearance of such screens is highly sensitive to the quality and placement of the glass microspheres used. Microspheres that are of incorrect size, are not spherical, or are broken, nicked, scratched, or otherwise defective can create a variety of visible defects, variously called graininess, scintillation, sparkles, speckle, punch through, or simply spots. These defects are particularly troubling when the screen is used, for example, as a computer monitor, where the need for seeing a high level of detail is likely to lead the user to scrutinize the screen closely, from a short distance, for long periods of time.
Generally, the size of the microspheres required for such products are less than about 150 .mu.m and for maximum "brightness" their index of refraction should be less than about 1.8, and preferably about 1.45 to about 1.75. Higher index microspheres can be employed as taught in U.S. Pat. No. 5,563,738 (Vance); however, to achieve similar brightness values special optical layers are required which adds additional processing steps and cost. It is also taught that it is "necessary to eliminate out-of-round, wrong-sized, and discolored microspheres" in order to obtain a uniform appearance.
A number of processes have been devised for the production of spherical glass bodies in small sizes. These generally involve the free suspension of particles in a hot zone for a time and at a temperature sufficient to permit each particle to be drawn into a spherical shape by surface tension. For economical commercial production of glass microspheres it is important that the viscosity of the glass be relatively low at a reasonable melting temperature (for example, no greater than about 1350.degree. C.). Generally, additions of alkali and fluorine are used to reduce the melting temperature; however, the use of fluorine creates an environmental concern as it is readily lost during the melting process and the addition of alkali typically results in microspheres that are hydrophobic and tend to clump and be poorly flowing.
U.S. Pat. No. 2,610,922 (Beck) describes glass compositions suitable for the production of glass microspheres with an index of refraction of 1.64 to 1.74. Compositions that are fluorine-free tend to form fiber when directly atomized from the melt; however, the use of fluorine in the glass results in hazardous emissions which are undesirable.
U.S. Pat. No. 5,716,706 (Morris) describes glass microspheres with a refractive index of 1.6 to 1.9. These glasses are designed to meet the refractive index, chemical durability, and strength requirements of pavement marking applications. These compositions do not readily form small microspheres (e.g., about 150 .mu.m or less) of acceptable quality (e.g., low levels of bubbles) due to the relatively high viscosity at useful microsphere forming temperatures (e.g., about 1350.degree. C.).
U.S. Pat. No. 3,306,757 (Duval d'Adrian) describes formulations that can be used to prepare glass microspheres in the desired refractive index range; however, these compositions either require excessive temperatures (e.g., greater than about 1350.degree. C.) or are of such a nature that they tend to form fibers when directly atomized from the melt.
U.S. Pat. No. 2,794,301 (Law et al.) describes free-flowing alkali metal oxide containing glass microspheres that are treated with an acidic gas vapor upon manufacture to insolubilize the surface alkali. This process creates undesirable environmental emissions which requires costly control.
Thus, there is a need for free flowing glass microspheres prepared from compositions that have a relatively low melting points and lend themselves to the economical manufacture of glass microspheres. Preferably, there is a need for glass forming compositions that have a low viscosity (e.g., that of vegetable oil) at temperatures no greater than about 1350.degree. C. and form microspheres with an index of refraction of no greater than about 1.70, which also have a low level of defects.